Voltage detector circuits and methods

ABSTRACT

A voltage detector includes a first node configured to have a first supply voltage, a second node configured to have a second supply voltage, and an output node. The voltage detector is configured to drive the output node to the first supply voltage in response to a difference between the first supply voltage and the second supply voltage exceeding a predetermined threshold voltage value.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.15/167,149, filed May 27, 2016, which is incorporated herein byreference in its entirety.

BACKGROUND

Integrated circuits (ICs) are potentially exposed to electricalover-stress (EOS) events from power surges and other sources that canimpact circuit reliability. During EOS events, voltage levels onconductors, or nodes, used to deliver power to circuits aresubstantially higher than normal voltage levels, so circuit componentscan be damaged. EOS concerns increase as IC dimensions shrink due to theincreased vulnerability of thin oxides and low junction breakdownvoltages.

Compared to transient electrostatic discharge (ESD) events with fastrise times and short durations, EOS events have slower rise times,higher energies, and/or wider ranges of pulse widths. Protectingcircuits from EOS events therefore entails approaches that can differfrom those used for ESD events. In most cases, ESD protection circuitsare built on an IC chip to address ESD threats within all manufacturingand packaging processes, while some discrete EOS or system ESDprotection elements are located outside the chip.

An EOS circuit is provided in some prior approaches by diode stringsthat protect sensitive circuit elements during EOS events but allowsignificant leakage currents during normal circuit operation.Polysilicon diodes used in some approaches need to be large to avoidsignificant heat generation during an EOS event.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other featuresand advantages will be apparent from the description, drawings, andclaims.

FIG. 1 is a diagram of a power clamp circuit, in accordance with someembodiments.

FIG. 2 is a diagram of a power clamp circuit, in accordance with someembodiments.

FIG. 3 is a diagram of voltage detectors, in accordance with someembodiments.

FIG. 4 is a diagram of drive circuits, in accordance with someembodiments.

FIG. 5 is a diagram of clamp circuits, in accordance with someembodiments.

FIG. 6 is a diagram of a transient detector, in accordance with someembodiments.

FIG. 7A-7C are diagrams of a power clamp circuits, in accordance withsome embodiments.

FIG. 8 is a flow chart of a method of operating a power clamp circuit,in accordance with some embodiments.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Embodiments, or examples, illustrated in the drawings are disclosedbelow using specific language. It will nevertheless be understood thatthe embodiments and examples are not intended to be limiting. Anyalterations and modifications in the disclosed embodiments, and anyfurther applications of the principles disclosed in this document arecontemplated as would normally occur to one of ordinary skill in thepertinent art.

In various embodiments, a power clamp circuit responds to a detected EOSevent by establishing a local current path between a first power nodeand a second power node. The local current path acts as a power clamp,thereby protecting other circuits connected to the power nodes from fullexposure to the energy delivered by the EOS event.

In various embodiments, a power clamp circuit includes a first nodeconfigured to have a first supply voltage and a second node configuredto have a second supply voltage. A voltage detector, including a firstoutput node, and a clamp circuit are coupled between the first node andthe second node. The voltage detector is configured to output a firstvoltage value to the first output node in response to a differencebetween the first supply voltage and the second supply voltage exceedinga predetermined threshold voltage value, and the clamp circuit isconfigured to establish a conduction path between the first node and thesecond node in response to the first voltage value. In some embodiments,a drive circuit coupled between the first node and the second node isconfigured to receive the first voltage value and output a controlvoltage to the clamp circuit.

In some embodiments, a transient detector including a second output nodeis also coupled between the first node and the second node. Thetransient detector is configured to output a second voltage value to thesecond output node in response to a transient difference between theslopes of rising edges of the first supply voltage and the second supplyvoltage exceeding a predetermined threshold change value. The drivecircuit is configured to output the control voltage to the clamp circuitin response to the first voltage value and the second voltage value.

FIG. 1 is a diagram of a power clamp circuit 100, in accordance withsome embodiments. Power clamp circuit 100 comprises a first supply nodeN1 and a second supply node N2. A voltage detector 110, a drive circuit120, and a clamp circuit 130 are coupled in parallel between first nodeN1 and second node N2. In some embodiments, an output node 112 ofvoltage detector 110 is coupled with an input node 122 of drive circuit120, and an output node 124 of drive circuit 120 is coupled with aninput node 132 of clamp circuit 130. In some embodiments, drive circuit120 is not present and output node 112 of voltage detector is coupledwith input node 132 of clamp circuit 130.

In some embodiments, the components of power clamp circuit 100,including voltage detector 110, clamp circuit 130, and, if present,drive circuit 120, are components of a single IC chip.

First supply node N1 is configured to have a first supply voltage. Insteady state operation, the first supply voltage has a first directcurrent (DC) voltage value. In some embodiments, the first DC voltagevalue is VDD of a functional circuit (not shown) coupled with firstsupply node N1. In some embodiments, the first DC voltage value is a VDDvalue of 1.8V. In some embodiments, the first DC voltage value is a VDDvalue of 0.8V. In various embodiments, a functional circuit coupled withfirst supply node N1 is a telecommunication, audio, processor, memory,power electronic system, application specific integrated circuit (ASIC),universal serial bus (USB) or other type interface, logic, signalprocessing, or similar type of circuit. In some embodiments, thefunctional circuit and power clamp circuit 100 are components of thesame IC chip.

Second supply node N2 is configured to have a second supply voltage. Insteady state operation, the second supply voltage has a second DCvoltage value different from the first DC voltage value. In someembodiments, the second DC voltage value is VSS of a functional circuitcoupled with first supply node N1 and second supply node N2. In someembodiments, the second DC voltage value is a ground reference voltageof a functional circuit coupled with first supply node N1 and secondsupply node N2.

In some embodiments, a first DC voltage value and/or a second DC voltagevalue include one or more non-DC voltage components, but would beunderstood by a person of ordinary skill in the art as being effectivelyDC voltages. For example, one or both of a first DC voltage value and asecond DC voltage value can include an alternating current (AC) noisesignal having a magnitude significantly smaller than the DC component ofthe first DC voltage value or the second DC voltage value.

Voltage detector 110 is configured to detect a difference between thefirst supply voltage on first supply node N1 and the second supplyvoltage on second supply node N2 in comparison with a predeterminedthreshold voltage value. Voltage detector 110 is configured to output anoutput voltage on output node 112 based on the comparison between thedifference and the predetermined threshold voltage value. If thedifference between the first supply voltage and the second supplyvoltage is less than or equal to the predetermined threshold voltagevalue, voltage detector 110 is configured to output a first outputvoltage value to output node 112. If the difference between the firstsupply voltage and the second supply voltage exceeds the predeterminedthreshold voltage value, voltage detector 110 is configured to output asecond output voltage value to output node 112 different from the firstoutput voltage value.

For an application of power clamp circuit 100 having a nominal first DCvoltage value and a nominal second DC voltage value during steady stateoperation, voltage detector 110 is configured to detect an EOS event byselecting a predetermined threshold voltage value larger than adifference between the nominal first DC voltage value and the nominalsecond DC voltage value. In operation, during a normal steady state orwhile normally powering up or powering down to/from steady state, thedifference between the first supply voltage value and the second supplyvoltage value is equal to or below the predetermined threshold voltagevalue, so voltage detector 110 outputs the first output voltage value.During an EOS event, in operation, the first supply voltage has a valuerelative to the second supply voltage that is above the predeterminedthreshold voltage value, so voltage detector 110 outputs the secondoutput voltage value.

In some embodiments, the first output voltage value represents a firstlogic value, e.g., high or low, and the second output voltage valuerepresents a second logic value, e.g., low or high. In some embodiments,voltage detector 110 is configured to generate one of the first outputvoltage value or the second output voltage value by driving output node112 to one of the first supply voltage or the second supply voltage. Insome embodiments, voltage detector 110 is configured to generate eitherthe first output voltage value or the second output voltage value bydriving output node 112 to one of the first supply voltage or the secondsupply voltage, and to generate the other of the first output voltagevalue or the second output voltage value by driving output node 112 tothe other one of the first supply voltage or the second supply voltage.

Non-limiting examples of circuits usable as voltage detector 110 arediscussed below with respect to FIG. 3.

Drive circuit 120 is configured to receive the output voltage from node112, generate an output voltage in response to the output voltage fromnode 112, and output the output voltage to output node 124. To outputthe output voltage, drive circuit 120 is configured to output currentsufficient to drive a load presented at input node 132 of clamp circuit130 so that the load has the output voltage within a predeterminedamount of time. In some embodiments, the load is a capacitive loadassociated with a gate terminal of a transistor of clamp circuit 130,and the current sufficient to drive the load is based on a switchingtime of the transistor.

In some embodiments, generating the output voltage comprises generatingthe output voltage having a logic value opposite a logic value of theoutput voltage from node 112. In some embodiments, drive circuit 120includes an inverter configured to generate an output voltage having alogic value opposite a logic value of the output voltage from node 112.In some embodiments, drive circuit 120 includes one or more output nodes(not shown) in addition to output node 124. A non-limiting example of acircuit usable as drive circuit 120 is discussed below with respect tocircuit 400A and FIG. 4

Clamp circuit 130 is configured to receive a control voltage andselectively establish a current path between first supply node N1 andsecond supply node N2. Clamp circuit 130 is configured to establish thecurrent path in response to the control voltage having a first controlvoltage value and to not establish the current path in response to thecontrol voltage having a second control voltage value.

Input node 132 of clamp circuit 130 is coupled with output node 124 ofdrive circuit 120 and the control voltage is the output voltage of drivecircuit 120 on output node 124. In some embodiments, drive circuit 120is not present, input node 132 of clamp circuit 130 is coupled withoutput node 112 of voltage detector 110, and the control voltage is theoutput voltage on output node 112.

In some embodiments, clamp circuit 130 includes a switch responsive tothe control voltage. In some embodiments, the switch is an n-type metaloxide semiconductor (NMOS) transistor, a gate of the NMOS transistor iscoupled with input node 132, and the current path includes a channel ofthe NMOS transistor. In some embodiments, the switch is a p-type metaloxide semiconductor (PMOS) transistor, a gate of the PMOS transistor iscoupled with input node 132, and the current path includes a channel ofthe PMOS transistor.

In some embodiments, clamp circuit 130 includes one or more input nodes(not shown) in addition to input node 132. Non-limiting examples ofcircuits usable as clamp circuit 130 are discussed below with respect toFIG. 5.

Voltage detector 110, drive circuit 120, if present, and clamp circuit130, are configured to establish a current path between first supplynode N1 and second supply node N2 in response to a difference betweenthe first supply voltage and the second supply voltage exceeding apredetermined threshold voltage value. Power clamp circuit 100 isthereby configured to, in operation, act as a voltage clamp in responseto an EOS event such that the effect of the EOS on other circuitscoupled with first supply node N1 and second supply node N2 issignificantly reduced in comparison with prior approaches.

FIG. 2 is a diagram of a power clamp circuit 200, in accordance withsome embodiments. Power clamp circuit 200 includes first supply node N1,second supply node N2, voltage detector 110, and clamp circuit 130,discussed above with respect to power clamp circuit 100. In contrast topower clamp circuit 100, power clamp circuit 200 does not include drivecircuit 120. Power clamp circuit 200 also includes transient detector240 and drive circuit 220 coupled in parallel between first supply nodeN1 and second supply node N2. Transient detector 240 includes an outputnode 242. Drive circuit 220 includes a first input node 222 coupled withoutput node 112 of voltage detector 110, a second input node 224 coupledwith output node 242, and an output node 226 coupled with input node 132of clamp circuit 130.

In some embodiments, the components of power clamp circuit 200,including voltage detector 110, clamp circuit 130, drive circuit 220,and transient detector 240, are components of a single IC chip. In someembodiments, power clamp circuit 200 and a functional circuit coupledwith first supply node N1 and second supply node N2 are components ofthe same IC chip.

Transient detector 240 is configured to detect a rate of change of thedifference between the first supply voltage on first supply node N1 andthe second supply voltage on second supply node N2. Transient detector240 is configured to output an output voltage on output node 242 basedon the rate of change of the difference. If the rate of change of thedifference between the first supply voltage and the second supplyvoltage remains equal to or below a predetermined threshold changevalue, transient detector 240 is configured to output a first outputvoltage value to output node 242. If the rate of change of thedifference between the first supply voltage and the second supplyvoltage exceeds the predetermined threshold change value, transientdetector 240 is configured to output a second output voltage value tooutput node 242 different from the first output voltage value.

By selecting a predetermined threshold change value larger than rates ofchange expected from normal operating conditions of functional circuitsconnected to supply node N1, transient detector 240 is configured todetect an ESD event. In operation, during transitional power on or offstates, or during steady state, the rate of change of the differencebetween the first DC voltage value and the second DC voltage value isequal to or below the predetermined threshold change value, so transientdetector 240 outputs the first output voltage value. During an ESDevent, in operation, the rate of change of the first supply voltagerelative to the second supply voltage exceeds the predeterminedthreshold change value, so transient detector 240 outputs the secondoutput voltage value.

In some embodiments, the first output voltage value represents a firstlogic value, e.g., high or low, and the second output voltage valuerepresents a second logic value, e.g., low or high. A non-limitingexample of a circuit usable as transient detector 240 is discussed belowwith respect to FIG. 6.

Drive circuit 220 is configured to generate the control voltage andoutput the control voltage to output node 226 in response to both theoutput voltage on node 112 of voltage detector 110 and the outputvoltage on node 242 of transient detector 240. Drive circuit 220 isconfigured to generate the control voltage having the first controlvoltage value if either the output voltage value on node 112 correspondsto detection of the supply voltage difference being above thepredetermined threshold voltage value or the output voltage value onnode 242 corresponds to detection of the rate of change of the supplyvoltage difference exceeding the predetermined threshold change value.Drive circuit 220 is configured to generate the control voltage havingthe second control voltage value if neither the output voltage value onnode 112 corresponds to detection of the supply voltage difference beingabove the predetermined threshold voltage value nor the output voltagevalue on node 242 corresponds to detection of the rate of change of thesupply voltage difference exceeding the predetermined threshold changevalue.

In some embodiments, drive circuit 220 includes one or more output nodes(not shown) in addition to output node 226. Non-limiting examples ofcircuits usable as drive circuit 220 are discussed below with respect tocircuit 400B, circuit 400C, and FIG. 4.

Voltage detector 110, transient detector 240, drive circuit 220, andclamp circuit 130 are configured to establish a current path betweenfirst supply node N1 and second supply node N2 in response to adifference between the first supply voltage and the second supplyvoltage exceeding a predetermined threshold voltage value or a rate ofchange of the difference between the first supply voltage and the secondsupply voltage exceeding a predetermined threshold change value. Powerclamp circuit 200 is thereby configured to, in operation, act as avoltage clamp in response to both EOS and ESD events such that theeffect on other circuits coupled with first supply node N1 and secondsupply node N2 is significantly reduced in comparison with priorapproaches.

FIG. 3 is a diagram of voltage detectors, in accordance with someembodiments. Each one of voltage detectors 300A, 300B, and 300C isusable as voltage detector 110 of power clamp circuit 100 or power clampcircuit 200, discussed above with respect to FIGS. 1 and 2,respectively.

Each one of voltage detectors 300A, 300B, and 300C is coupled betweenfirst supply node N1 and second supply node N2. In each voltagedetector, PMOS transistors T1 and T2 have source terminals electricallyconnected to first supply node N1. A gate terminal of transistor T2 iselectrically connected to a drain terminal of transistor T1 at a node A,and a gate terminal of transistor T1 is electrically connected to adrain terminal of transistor T2 at an output node C. In someembodiments, output node C corresponds to output node 112 of voltagedetector 110.

In each voltage detector, a diode D1 has an anode electrically connectedto node A and a cathode electrically connected to a node B. In someembodiments, diode D1 is a transistor configured as a diode. An NMOStransistor T3 has a drain terminal electrically connected to output nodeC, a source terminal electrically connected to second supply node N2,and a gate terminal electrically connected to node B. A resistor R1 hasa first terminal electrically connected to output node C and a secondterminal electrically connected to second supply node N2.

In each voltage detector, one or more elements are coupled between nodeB and second supply node N2. In voltage detector 300A, the elementscoupled between node B and second supply node N2 are a diode D2 inseries with a diode D3. An anode of diode D2 is electrically connectedto node B and a cathode of diode D3 is electrically connected to secondsupply node N2.

In voltage detector 300B, the elements coupled between node B and secondsupply node N2 are diode D2 in series with an NMOS transistor T4. Theanode of diode D2 is electrically connected to node B. NMOS transistorT4 is configured as a diode with each of a gate terminal and drainterminal electrically connected to a cathode of diode D2 and a sourceterminal electrically connected to second supply node N2.

In voltage detector 300C, the elements coupled between node B and secondsupply node N2 are diode D2 in series with PMOS transistors T5 and T6.The anode of diode D2 is electrically connected to node B. Eachtransistor of the series of PMOS transistors is configured as a diodeand is coupled between the cathode of diode D2 and second supply nodeN2. A source terminal of transistor T5 is electrically connected to thecathode of diode D2, and each of a drain terminal of transistor T6 and agate terminal of transistor T6 is electrically connected to secondsupply node N2.

In each one of voltage detector 300A, voltage detector 300B, and voltagedetector 300C, the two or more elements coupled between node B andsecond supply node N2 are chosen so that a voltage at node A sufficientto forward bias diode D1 and the one or more elements is greater thanthe first DC voltage value on first supply node N1 relative to thesecond DC voltage value on second supply node N2 during steady stateoperation or while powering up or powering down.

For each voltage detector, during steady state operation of functionalcircuits connected to supply node N1, current through resistor R1 issubstantially zero and second supply node N2 drives output supply node Cto the second DC voltage value, thereby generating the second DC voltagevalue on output node C. The gate terminal of transistor T1 thereforealso has the second DC voltage value. Because the source terminal oftransistor T1 has the first DC voltage value, transistor T1 is switchedon and node A has the first DC voltage value.

Because both node A and first supply node N1 have the first DC voltagevalue during steady state, transistor T2 is switched off, therebypreventing current from flowing from first supply node N1 to secondsupply node N2 through resistor R1. Because node A has the first DCvoltage value equal to or less than a voltage sufficient to forward biasdiode D1 and the two or more elements coupled between node B and secondsupply node N2, diode D1 and the two or more elements are switched offand no significant leakage current flows between first supply node N1and second supply node N2.

Each of voltage detector 300A, voltage detector 300B, and voltagedetector 300C is thereby configured so that, in steady state operation,output node C is driven by second supply node N2 so that the second DCvoltage value is generated at output node C.

In operation, a first supply voltage value above a steady state valuepulls the voltage at node A toward a value that forward biases diode D1and the two or more elements coupled between node B and second supplynode N2. This forward-bias value defines a clamp, or reference, voltagevalue for the gate terminal of transistor T2 electrically connected tonode A. Because the source terminal of transistor T2 is electricallyconnected to first supply node N1, a first supply voltage value thatexceeds the reference voltage value by at least a threshold voltage oftransistor T2 causes transistor T2 to be switched on.

Transistor T2 is selected to have a greater drive capacity than thedrive capacity of transistor T3. When transistor T2 is switched on,output node C is therefore driven toward the first supply voltage valueon first supply node N1 instead of the second supply voltage value onsecond supply node N2. Output node C being driven toward the firstsupply voltage value causes transistor T1 to be switched off, therebycausing each of node A and node B to have the second supply voltagevalue so that transistor T3 is switched off. With transistor T2 switchedon and transistor T3 switched off, output node C is driven by firstsupply node N1, and the first supply voltage value is generated onoutput node C.

Selection of the elements coupled between node A and second supply nodeN2 determines the reference voltage value of node A and the gateterminal of transistor T2. With the source terminal of transistor T2electrically connected to first supply node N1, the reference voltageand the threshold voltage of transistor T2 form the basis for apredetermined threshold voltage value at which each voltage detector, inoperation, changes the value of the voltage generated at output node C.In some embodiments, transistor T2 has a threshold voltage that rangesfrom 0.5 V-1.5 V.

In voltage detector 300A, the series of diodes D1, D2, and D3 defines areference voltage value for node A based on three forward biased diodes.In some embodiments, each of diodes D1, D2, and D3 has a forward biasthreshold of 0.6 V-0.7 V and the reference voltage value ranges from 1.8V-2.1 V. In some embodiments, the predetermined threshold voltage valuebased on the reference voltage value and the threshold voltage oftransistor T2 ranges from 2.3 V-3.6 V. In various embodiments, voltagedetector 300A is a component of a power clamp circuit in which a firstDC voltage value on first supply node N1 and a second DC voltage valueon second supply node N2 differ by 0.8 V or 1.8 V in steady stateoperation.

In voltage detector 300B, the series of diode D1, D2, and transistor T4defines a reference voltage value for node A based on two forward biaseddiodes and one transistor configured as a diode. In some embodiments,each of diodes D1 and D2 and transistor T4 has a forward bias thresholdof 0.6 V-0.7 V and the reference voltage value ranges from 1.8 V-2.1 V.In some embodiments, the predetermined threshold voltage value based onthe reference voltage value and the threshold voltage of transistor T2ranges from 2.3 V-3.6 V. In various embodiments, voltage detector 300Bis a component of a power clamp circuit in which a first DC voltagevalue on first supply node N1 and a second DC voltage value on secondsupply node N2 differ by 0.8 V or 1.8 V in steady state operation.

In voltage detector 300C, the series of diode D1, D2, and transistors T5and T6 defines a reference voltage value for node A based on two forwardbiased diodes and two transistors configured as diodes. In someembodiments, each of diodes D1 and D2 and transistors T5 and T6 has aforward bias threshold of 0.6 V-0.7 V and the reference voltage valueranges from 2.4 V-2.8 V. In some embodiments, the predeterminedthreshold voltage value based on the reference voltage value and thethreshold voltage of transistor T2 ranges from 2.9 V-4.3 V. In variousembodiments, voltage detector 300C is a component of a power clampcircuit in which a first DC voltage value on first supply node N1 and asecond DC voltage value on second supply node N2 differ by 0.8 V or 1.8V in steady state operation.

Voltage detectors 300A, 300B, and 300C are non-limiting examples ofvoltage detectors usable as voltage detector 110 in power clamp circuit100 or power clamp circuit 200. Other voltage detectors having otherconfigurations and other ranges of predetermined threshold voltagevalues are within the scope of the present disclosure.

FIG. 4 is a diagram of drive circuits 400A and 400B, in accordance withsome embodiments. Each of drive circuit 400A and drive circuit 400B iscoupled between first supply node N1 and second supply node N2.

Drive circuit 400A includes a single input node D and a single outputnode E. Drive circuit 400A is usable as drive circuit 120 of power clampcircuit 100, discussed above with respect to FIG. 1. In someembodiments, input node D corresponds to input node 122 of drive circuit120 and output node E corresponds to output node 124 of drive circuit120.

Drive circuit 400A is configured as an inverter including a PMOStransistor T7 in series with an NMOS transistor T8. A source terminal oftransistor T7 is electrically connected to first supply node N1 and asource terminal of transistor T8 is electrically connected to secondsupply node N2. Input node D is electrically connected to a gateterminal of transistor T7 and to a gate terminal of transistor T8. Adrain terminal of transistor T7 is electrically connected to a drainterminal of transistor T8 and to output node E.

Drive circuit 400A is thereby configured so that, in operation, avoltage on input node D having a high logic value causes a voltagehaving a low logic value to be generated on output node E, and a voltageon input node D having a low logic value causes a voltage having a highlogic value to be generated on output node E.

Drive circuit 400B includes a first input node F, a second input node G,and a single output node H. Drive circuit 400B is usable as drivecircuit 220 of power clamp circuit 200, discussed above with respect toFIG. 2. In some embodiments, input node F corresponds to input node 222of drive circuit 220, input node G corresponds to input node 224 ofdrive circuit 220, and output node H corresponds to output node 226 ofdrive circuit 220.

Drive circuit 400B is configured as a NOR gate including a PMOStransistor T9 in series with a PMOS transistor T10 and with a parallelcombination of an NMOS transistor T11 and an NMOS transistor T12. Asource terminal of transistor T9 is electrically connected to firstsupply node N1, a source terminal of transistor T11 is electricallyconnected to second supply node N2, and a source terminal of transistorT12 is electrically connected to second supply node N2.

Input node F is electrically connected to a gate terminal of transistorT9 and to a gate terminal of transistor T11. Input node G iselectrically connected to a gate terminal of transistor T10 and to agate terminal of transistor T12. A drain terminal of transistor T9 iselectrically connected to a source terminal of transistor T10. A drainterminal of transistor T10, a drain terminal of transistor T11, and adrain terminal of transistor T12 are electrically connected to outputnode H.

Drive circuit 400B is thereby configured so that, in operation, avoltage on input node F having a high logic value and/or a voltage oninput node G having a high logic value causes a voltage having a lowlogic value to be generated on output node H. In operation, a voltage oninput node F having a low logic value and a voltage on input node Ghaving a low logic value causes a voltage having a high logic value tobe generated on output node H.

Drive circuits 400A and 400B are non-limiting examples of drive circuitsusable in power clamp circuit 100 or power clamp circuit 200. Otherdrive circuits having other configurations are within the scope of thepresent disclosure. For example, in some embodiments, a drive circuitincludes more than two input nodes and/or more than one output node. Insome embodiments, a drive circuit includes one or more logic gates, e.g.OR, AND, or NAND gates, as alternatives or additions to the logic gatesof drive circuits 400A and 400B.

FIG. 5 is a diagram of clamp circuits, in accordance with someembodiments. Each of clamp circuits 500A, 500B, 500C, and 500D is usableas clamp circuit 130 of power clamp circuit 100 or power clamp circuit200, discussed above with respect to FIGS. 1 and 2, respectively.

Each of clamp circuits 500A, 500B, 500C, and 500D is coupled betweenfirst supply node N1 and second supply node N2 and is configured toselectively establish a current path between first supply node N1 andsecond supply node N2 responsive to a control voltage at one or moreinput nodes.

Clamp circuit 500A includes a PMOS transistor T13 having a sourceterminal electrically connected to first supply node N1, a drainterminal electrically connected to second supply node N2, and a gateterminal electrically connected to an input node I. In some embodiments,input node I corresponds to input node 132 of clamp circuit 130.

Clamp circuit 500B includes an NMOS transistor T14 having a drainterminal electrically connected to first supply node N1, a sourceterminal electrically connected to second supply node N2, and a gateterminal electrically connected to an input node J. In some embodiments,input node J corresponds to input node 132 of clamp circuit 130.

Clamp circuit 500C includes a PMOS transistor T15 in series with diodesD4, D5, and D6. An anode of diode D4 is electrically connected to firstsupply node N1, a drain terminal of transistor T15 is electricallyconnected to second supply node N2, and diodes D5 and D6 areelectrically connected in series between diode D4 and transistor T15. Agate terminal of transistor T15 is electrically connected to an inputnode K. In some embodiments, input node K corresponds to input node 132of clamp circuit 130.

Clamp circuit 500D includes a PMOS transistor T16 having a sourceterminal electrically connected to first supply node N1 in series with aPMOS transistor T17 having a drain terminal electrically connected tosecond supply node N2. A gate terminal of transistor T16 is electricallyconnected to a first input node L and a gate terminal of transistor T17is electrically connected to a second input node M.

Clamp circuits 500A, 500B, 500C, and 500D are non-limiting examples ofclamp circuits usable as clamp circuit 130 in power clamp circuit 100 orpower clamp circuit 200. Other clamp circuits having otherconfigurations for selectively establishing a current path between firstsupply node N1 and second supply node N2 responsive to a control voltageat one or more input nodes are within the scope of the presentdisclosure.

FIG. 6 is a diagram of a transient detector 600, in accordance with someembodiments. Transient detector 600 is usable as transient detector 240of power clamp circuit 200, discussed above with respect to FIG. 2.

Transient detector 600 is coupled between first supply node N1 andsecond supply node N2 and includes a transistor T18 in series with aresistor R2. Transistor T18 is configured as a capacitor by having asource terminal and a drain terminal electrically connected to firstsupply node N1 such that transistor T18 and resistor R2 form an RCcircuit. A gate terminal of transistor T18 is electrically connected toa first terminal of resistor R2 and to an output node N, and a secondterminal of resistor R2 is electrically connected to second supply nodeN2. In some embodiments, output node N corresponds to output node 242 oftransient detector 240.

In steady state operation, a capacitance of transistor T18 is charged sothat the source and drain terminals of transistor T18 have the firstsupply voltage present on first supply node N1. No significant currentflows through resistor R2, so output node N has the second supplyvoltage present on second supply node N2. In operation, a change in thefirst supply voltage having a rate of change that exceeds apredetermined threshold change value, as discussed below, causes node Nto be coupled with first supply node N1 so that the first supply voltageis presented on node N.

Selection of a capacitance value of transistor T18 in combination with aresistance value of resistor R2 causes transient detector 600 to have apredetermined threshold change value. A rate of change of the differencebetween the first supply voltage and the second supply voltage thatexceeds the predetermined threshold change value causes transientdetector 600 to generate the first supply voltage on output node N. Arate of change of the difference between the first supply voltage andthe second supply voltage that is equal to or less than thepredetermined threshold change value causes transient detector 600 togenerate the second supply voltage on output node N.

Transient detector 600 is a non-limiting example of a transient detectorusable as transient detector 240 in power clamp circuit 200. Othertransient detectors having other configurations are within the scope ofthe present disclosure.

FIG. 7A is a diagram of a power clamp circuit 700A, in accordance withsome embodiments. Power clamp circuit 700A includes first supply nodeN1, second supply node N2, voltage detector 110, clamp circuit 130, and,in some embodiments, transient detector 240, each of which is discussedabove with respect to power clamp circuit 100 and power clamp circuit200. In contrast to power clamp circuit 100, power clamp circuit 700Adoes not include drive circuit 120. Power clamp circuit 700A alsoincludes a third supply node N3, a voltage detector 710A coupled betweenfirst supply node N1 and third supply node N3, and a drive circuit 720coupled between first supply node N1 and second supply node N2. Voltagedetector 710A includes an output node 712A, and drive circuit 720includes a first input node 721 coupled with output node 712A of voltagedetector 710A, a second input node 722 coupled with output node 242 oftransient detector 240, a third input node 723 coupled with output node112 of voltage detector 110, and an output node 726 coupled with inputnode 132 of clamp circuit 130. In some embodiments, power clamp circuit700A does not include transient detector 240, output node 242 oftransient detector 240, or second input node 722 of drive circuit 720.

Third supply node N3 is configured to have a third supply voltage. Insteady state operation, the third supply voltage has a third DC voltagevalue between the first DC voltage value and the second DC voltagevalue, discussed above with respect to power clamp circuit 100. In someembodiments, the first DC voltage value is VDDIO of a first functionalcircuit coupled with first supply node N1 and second supply node N2, andthe third DC voltage value is VDDCORE of a second functional circuitcoupled with third supply node N3 and second supply node N2. In someembodiments, VDDIO has a nominal value of 1.8 V. In some embodiments,VDDCORE has a nominal value of 0.8 V.

Voltage detector 710A is configured to operate similarly to voltagedetector 110, described above with respect to power clamp circuit 100,but operates based on a second predetermined threshold voltage valuedifferent from the first predetermined threshold voltage value ofvoltage detector 110. Accordingly, if a difference between the firstsupply voltage and the third supply voltage is equal to or less than thesecond predetermined threshold voltage value, voltage detector 710A isconfigured to output a first output voltage value to output node 712A.If the difference between the first supply voltage and the third supplyvoltage exceeds the second predetermined threshold voltage value,voltage detector 710A is configured to output a second output voltagevalue to output node 712A different from the first output voltage value.

By selecting the second predetermined threshold voltage value largerthan a difference between the first DC voltage value and the third DCvoltage value, voltage detector 710A is configured to detect an EOSevent. In operation, during steady state or while powering up orpowering down, the difference between the first DC voltage value and thethird DC voltage value is equal to or below the second predeterminedthreshold voltage value, so voltage detector 710A outputs the firstoutput voltage value. During an EOS event, in operation, the firstsupply voltage can have a value relative to the third supply voltagethat is above the second predetermined threshold voltage value, sovoltage detector 710A outputs the second output voltage value.

Drive circuit 720 is configured to generate the control voltage onoutput node 726 in response to the output voltage on node 712A ofvoltage detector 710A, the output voltage on node 242 of transientdetector 240, and the output voltage on node 112 of voltage detector110. Drive circuit 720 is configured to generate the control voltagehaving the first control voltage value if the output voltage value onnode 112 corresponds to detection of a first supply voltage differenceabove the first predetermined threshold voltage value, if the outputvoltage value on node 242 corresponds to detection of the rate of changeof the first supply voltage difference exceeding the threshold changevalue, or if the output voltage value on node 712A corresponds todetection of a second supply voltage difference above the secondthreshold voltage value. Drive circuit 720 is configured to generate thecontrol voltage having the second control voltage value if the outputvoltage value on node 112 corresponds to detection of a first supplyvoltage difference equal to or below the first predetermined thresholdvoltage value, the output voltage value on node 242, corresponds todetection of the rate of change of the first supply voltage differencebeing equal to or below the threshold change value, and the outputvoltage value on node 712A corresponds to detection of a second supplyvoltage difference equal to or below the second threshold voltage value.

In some embodiments, power clamp circuit 700A does not include transientdetector 240, and drive circuit 720 is configured to generate thecontrol voltage on output node 726 in response to the output voltage onnode 712A of voltage detector 710A and the output voltage on node 112 ofvoltage detector 110. In some embodiments, drive circuit 720 isconfigured to generate the control voltage having the first controlvoltage value if the output voltage value on node 112 corresponds todetection of a first supply voltage difference above the firstpredetermined threshold voltage value or if the output voltage value onnode 712A corresponds to detection of a second supply voltage differenceabove the second threshold voltage value. In some embodiments, drivecircuit 720 is configured to generate the control voltage having thesecond control voltage value if the output voltage value on node 112corresponds to detection of a first supply voltage difference equal toor below the first predetermined threshold voltage value and the outputvoltage value on node 712A corresponds to detection of a second supplyvoltage difference equal to or below the second threshold voltage value.

FIG. 7B is a diagram of a power clamp circuit 700B, in accordance withsome embodiments. Power clamp circuit 700B includes first supply nodeN1, second supply node N2, third supply node N3, voltage detector 110,drive circuit 720, clamp circuit 130, and, in some embodiments,transient detector 240, each of which is discussed above with respect topower clamp circuit 700A. Instead of voltage detector 710A, power clampcircuit 700B includes a voltage detector 710B coupled between thirdsupply node N3 and second supply node N2. Voltage detector 710B includesan output node 712B, and drive circuit 720 includes first input node 721coupled with output node 242 of transient detector 240, second inputnode 722 coupled with output node 112 of voltage detector 110, thirdinput node 723 coupled with output node 712B of voltage detector 710B,and output node 726 coupled with input node 132 of clamp circuit 130. Insome embodiments, power clamp circuit 700B does not include transientdetector 240, output node 242 of transient detector 240, or second inputnode 722 of drive circuit 720.

Voltage detector 710B is configured to operate similarly to voltagedetector 110, described above with respect to power clamp circuit 100,but operates based on a third predetermined threshold voltage valuedifferent from the first predetermined threshold voltage value ofvoltage detector 110. Accordingly, if a difference between the thirdsupply voltage and the second supply voltage is equal to or less thanthe third predetermined threshold voltage value, voltage detector 710Bis configured to output a first output voltage value to output node712B. If the difference between the third supply voltage and the secondsupply voltage exceeds the third predetermined threshold voltage value,voltage detector 710B is configured to output a second output voltagevalue to output node 712B different from the first output voltage value.

In some embodiments, the third predetermined threshold voltage value isdifferent from the second predetermined threshold voltage value. In someembodiments, the third predetermined threshold voltage value is the sameas the second predetermined threshold voltage value.

FIG. 7C is a diagram of a power clamp circuit 700C, in accordance withsome embodiments. Power clamp circuit 700C includes first supply nodeN1, second supply node N2, third supply node N3, voltage detector 110,voltage detector 710A, voltage detector 710B, clamp circuit 130, and, insome embodiments, transient detector 240, each of which is discussedabove with respect to power clamp circuits 700A and 700B. Instead ofdrive circuit 720, power clamp circuit 700C includes drive circuit 725coupled between first supply node N1 and second supply node N2. Drivecircuit 725 includes first input node 721 coupled with output node 710Aof voltage detector 710A, second input node 722 coupled with output node242 of transient detector 240, third input node 723 coupled with outputnode 112 of voltage detector 110, fourth input node 724 coupled withoutput node 712B of voltage detector 710B, and output node 726 coupledwith input node 132 of clamp circuit 130. In some embodiments, powerclamp circuit 700C does not include transient detector 240, output node242 of transient detector 240, or second input node 722 of drive circuit725.

Drive circuit 725 is configured to generate the control voltage onoutput node 726 in response to the output voltage on node 712A ofvoltage detector 710A, the output voltage on node 242 of transientdetector 240, the output voltage on node 112 of voltage detector 110,and the output voltage on node 712B of voltage detector 710B. Drivecircuit 725 is configured to generate the control voltage having thefirst control voltage value if the output voltage value on node 112corresponds to detection of a first supply voltage difference above thefirst predetermined threshold voltage value, if the output voltage valueon node 242 corresponds to detection of the rate of change of the firstsupply voltage difference exceeding the threshold change value, if theoutput voltage value on node 712A corresponds to detection of a secondsupply voltage difference above the second threshold voltage value, orif the output voltage value on node 712B corresponds to detection of athird supply voltage difference above the third threshold voltage value.Drive circuit 725 is configured to generate the control voltage havingthe second control voltage value if the output voltage value on node 112corresponds to detection of the first supply voltage difference equal toor below the first predetermined threshold voltage value, the outputvoltage value on node 242 corresponds to detection of the rate of changeof the first supply voltage difference being equal to or below thethreshold change value, the output voltage value on node 712Acorresponds to detection of the second supply voltage difference equalto or below the second threshold voltage value, and the output voltagevalue on node 712B corresponds to detection of the third supply voltagedifference equal to or below the third threshold voltage value.

In some embodiments, power clamp circuit 700A does not include transientdetector 240, and drive circuit 725 is configured to generate thecontrol voltage on output node 726 in response to the output voltage onnode 712A of voltage detector 710A, the output voltage on node 112 ofvoltage detector 110, and the output voltage on node 712B of voltagedetector 710B. In some embodiments, drive circuit 725 is configured togenerate the control voltage having the first control voltage value ifthe output voltage value on node 112 corresponds to detection of a firstsupply voltage difference above the first predetermined thresholdvoltage value, if the output voltage value on node 712A corresponds todetection of a second supply voltage difference above the secondthreshold voltage value, or if the output voltage value on node 712Bcorresponds to detection of a third supply voltage difference above thethird threshold voltage value. In some embodiments, drive circuit 725 isconfigured to generate the control voltage having the second controlvoltage value if the output voltage value on node 112 corresponds todetection of the first supply voltage difference equal to or below thefirst predetermined threshold voltage value, the output voltage value onnode 712A corresponds to detection of the second supply voltagedifference equal to or below the second threshold voltage value, and theoutput voltage value on node 712B corresponds to detection of the thirdsupply voltage difference equal to or below the third threshold voltagevalue.

Each of power clamp circuits 700A, 700B, and 700C is configured torespond to EOS and ESD events similarly to power clamp circuit 100 andpower clamp circuit 200. By including voltage detector 710A and/orvoltage detector 710B and drive circuit 720 or drive circuit 725responsive to voltage detector 710A and/or voltage detector 710B, eachof power clamp circuits 700A, 700B, and 700C is configured to respond toan EOS event on a circuit including third supply node N3.

Power clamp circuits 700A, 700B, and 700C are non-limiting examples ofpower clamp circuits for circuit configurations including three supplynodes. Other power clamp circuits having similar configurations based onvoltage detectors for circuit configurations including more than threesupply nodes are within the scope of the present disclosure.

FIG. 8 is a flow chart of a method 800 of operating a power clampcircuit, in accordance with some embodiments. Method 800 is capable ofbeing performed with any of power clamp circuits 100, 200, 700A, 700B,and 700C, as discussed above.

At operation 802, a voltage detector detects a first difference betweena first supply node voltage and a second supply node voltage exceeding afirst predetermined threshold voltage value. In various embodiments, thevoltage detector is one of voltage detectors 110, 100A, 300B, or 300Cdescribed above with respect to power clamp circuits 100, 200, 700A,700B, and 700C. In various embodiments, the first supply node voltage isa supply node voltage on first supply node N1 and the second supply nodevoltage is a supply node voltage on second supply node N2 describedabove with respect to power clamp circuits 100, 200, 700A, 700B, and700C.

At operation 804, in response to the detecting the first differenceexceeding the first predetermined threshold value, the voltage detectorgenerates a first output voltage value at a first output node of thevoltage detector. In some embodiments, generating the first outputvoltage value comprises driving the first output node to the firstsupply node voltage. In some embodiments, a first supply node having thefirst supply node voltage is first supply node N1. In some embodiments,driving the first output node to the first supply node voltage compriseselectrically connecting the first output node to the first supply nodewith a switch. In some embodiments, driving the first output node to thefirst supply node voltage comprises electrically connecting the firstoutput node to the first supply node with a PMOS transistor.

At operation 806, in some embodiments, a drive circuit receives thefirst output voltage value from the voltage detector. In variousembodiments, the first output voltage value is received by one of drivecircuits 120, 220, 400A, or 400B described above with respect to powerclamp circuits 100, 200, 700A, 700B, and 700C.

At operation 808, in some embodiments, the drive circuit outputs acontrol voltage to the clamp circuit, the control voltage having acontrol voltage value based on the first output voltage value.

At operation 810, based on the first output voltage value at the firstoutput node, a clamp circuit establishes a conduction path between thefirst supply node and a second supply node having the second supply nodevoltage. In some embodiments, the clamp circuit receives the firstoutput voltage value from the voltage detector. In some embodiments, theclamp circuit receives the control voltage value from the drive circuit.In various embodiments, establishing the conduction path comprisesestablishing the conduction path with one of clamp circuits 130, 500A,500B, 500C, or 500D described above with respect to power clamp circuits100, 200, 700A, 700B, and 700C.

At operation 812, in some embodiments, a transient detector detects arate of change of the difference between the first supply node voltageand the second supply node voltage exceeding a predetermined thresholdchange value. In various embodiments, the transient detector is one oftransient detectors, 240 or 600 described above with respect to powerclamp circuits 200, 700A, 700B, and 700C.

At operation 814, in some embodiments, in response to the detecting therate of change exceeding the predetermined threshold change value, thetransient detector outputs a second output voltage value on a secondoutput node of the transient detector.

At operation 816, in some embodiments, the drive circuit receives thesecond output voltage value from the second output node of the transientdetector.

At operation 818, in some embodiments, the drive circuit outputs thecontrol voltage having the control voltage value further based on thesecond output voltage value.

At operation 820, in some embodiments, a second voltage detector detectsa second difference between a third power supply node voltage and one ofthe first supply node voltage or the second supply node voltageexceeding a second predetermined threshold voltage value. In variousembodiments, the second voltage detector is one of voltage detectors710A or 710B and the third supply node voltage is the third supply nodevoltage on third supply node N3, described above with respect to powerclamp circuits 700A, 700B, and 700C.

At operation 822, in some embodiments, in response to the detecting thesecond difference, the second voltage detector outputs a third outputvoltage value on a third output node of the second voltage detector.

At operation 824, in some embodiments, the drive circuit receives thethird output voltage value from the third output node of the secondvoltage detector.

At operation 826, in some embodiments, the drive circuit outputs thecontrol voltage having the control voltage value further based on thethird output voltage value.

In some embodiments, a voltage detector includes a first node configuredto have a first supply voltage, a second node configured to have asecond supply voltage, and an output node. The voltage detector isconfigured to drive the output node to the first supply voltage inresponse to a difference between the first supply voltage and the secondsupply voltage exceeding a predetermined threshold voltage value.

In some embodiments, a voltage detection circuit includes a first nodeconfigured to have a first supply voltage, a second node configured tohave a second supply voltage, a third node configured to have a thirdsupply voltage, a first voltage detector coupled between the first nodeand the second node, the first voltage detector being configured todrive a first output node to the first supply voltage in response to adifference between the first supply voltage and the second supplyvoltage exceeding a first predetermined threshold voltage value, and asecond voltage detector. The second voltage detector is either coupledbetween the first node and the third node and configured to drive asecond output node to the first supply voltage in response to adifference between the first supply voltage and the third supply voltageexceeding a second predetermined threshold voltage, or is coupledbetween the third node and the second node and configured to drive thesecond output node to the third supply voltage in response to adifference between the third supply voltage and the second supplyvoltage exceeding a third predetermined threshold voltage.

In some embodiments, a method of operating a voltage detector circuitincludes detecting a difference between a first supply voltage on afirst supply node and a second supply voltage on a second supply nodeexceeding a first predetermined threshold voltage value, and, inresponse to the detecting the difference exceeding the firstpredetermined threshold value, generating a first output voltage valueby driving a first output node of the voltage detector to the firstsupply voltage.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A voltage detector comprising: a first nodeconfigured to have a first supply voltage; a second node configured tohave a second supply voltage; and an output node, wherein the voltagedetector is configured to drive the output node to the first supplyvoltage in response to a difference between the first supply voltage andthe second supply voltage exceeding a predetermined threshold voltagevalue.
 2. The voltage detector of claim 1, further comprising a switchcoupled between the first node and the output node.
 3. The voltagedetector of claim 2, wherein the switch comprises a p-type metal oxidesemiconductor (PMOS) transistor.
 4. The voltage detector of claim 2,further comprising a reference node configured to have a referencevoltage value based on the predetermined threshold voltage value,wherein the switch comprises a control terminal coupled with thereference node.
 5. The voltage detector of claim 4, wherein thereference node is configured to have the reference voltage value greaterthan a steady state value of the first supply voltage.
 6. The voltagedetector of claim 4, wherein the reference node is configured to havethe reference voltage value of the predetermined threshold voltage valueminus a threshold voltage of the switch.
 7. The voltage detector ofclaim 4, further comprising a diode coupled between the reference nodeand the second node, wherein the reference voltage value is based on aforward bias threshold of the diode.
 8. The voltage detector of claim 7,further comprising at least one of another diode or a transistorconfigured as a diode in series with the diode, wherein the referencevoltage value is further based on a forward bias threshold of the atleast one of the another diode or the transistor configured as a diode.9. The voltage detector of claim 1, wherein the voltage detector isconfigured to drive the output node to the second supply voltage inresponse to the difference between the first supply voltage and thesecond supply voltage being less than or equal to the predeterminedthreshold voltage value.
 10. The voltage detector of claim 1, wherein:the first supply voltage has a predetermined steady state direct current(DC) value corresponding to a functional circuit coupled with the firstnode, the second supply voltage is a ground reference voltage of thefunctional circuit, and the voltage detector and the functional circuitare components of a single integrated circuit (IC) chip.
 11. A voltagedetection circuit comprising: a first node configured to have a firstsupply voltage; a second node configured to have a second supplyvoltage; a third node configured to have a third supply voltage; a firstvoltage detector coupled between the first node and the second node, thefirst voltage detector being configured to drive a first output node tothe first supply voltage in response to a difference between the firstsupply voltage and the second supply voltage exceeding a firstpredetermined threshold voltage value; and a second voltage detectoreither: coupled between the first node and the third node and configuredto drive a second output node to the first supply voltage in response toa difference between the first supply voltage and the third supplyvoltage exceeding a second predetermined threshold voltage, or coupledbetween the third node and the second node and configured to drive thesecond output node to the third supply voltage in response to adifference between the third supply voltage and the second supplyvoltage exceeding a third predetermined threshold voltage.
 12. Thevoltage detection circuit of claim 11, wherein: the first supply voltagehas a first predetermined steady state direct current (DC) valuecorresponding to a first functional circuit coupled with the first node,the second supply voltage is a ground reference voltage, and the thirdsupply voltage has a second predetermined steady state DC value betweenthe first DC value and the ground reference voltage, and correspondingto a second functional circuit coupled with the third node.
 13. Thevoltage detection circuit of claim 12, wherein the first voltagedetector, the second voltage detector, the first functional circuit, andthe second functional circuit are components of a single integratedcircuit (IC) chip.
 14. The voltage detection circuit of claim 11,wherein: the second voltage detector is coupled between the first nodeand the third node, and the voltage detection circuit comprises a thirdvoltage detector coupled between the third node and the second node andconfigured to drive a third output node to the third supply voltage inresponse to the difference between the third supply voltage and thesecond supply voltage exceeding the third predetermined thresholdvoltage.
 15. A method of operating a voltage detector circuit, themethod comprising: detecting a difference between a first supply voltageon a first supply node and a second supply voltage on a second supplynode exceeding a first predetermined threshold voltage value; and inresponse to the detecting the difference exceeding the firstpredetermined threshold value, generating a first output voltage valueby driving a first output node of the voltage detector to the firstsupply voltage.
 16. The method of claim 15, wherein detecting thedifference exceeding the predetermined threshold voltage value comprisesforward biasing a diode to define a reference voltage value greater thana steady state value of the first supply voltage.
 17. The method ofclaim 16, wherein detecting the difference exceeding the predeterminedthreshold voltage value further comprises the first supply voltageexceeding the reference voltage value by a threshold voltage of a switchcoupled between an anode of the diode and the first supply node.
 18. Themethod of claim 15, wherein driving the first output node of the voltagedetector to the first supply voltage comprises turning on a p-type metaloxide semiconductor (PMOS) transistor coupled between the first outputnode and the first supply node.
 19. The method of claim 15, furthercomprising, in response to detecting the difference being less than orequal to the first predetermined threshold value, generating a secondoutput voltage value by driving the first output node of the voltagedetector to the second supply voltage.
 20. The method of claim 15,further comprising at least one of: in response to detecting adifference between the first supply voltage and a third supply voltageon a third supply node exceeding a second predetermined thresholdvoltage value, generating a second output voltage value by driving asecond output node of the voltage detector to the first supply voltage;or in response to detecting a difference between a third supply voltageon a third supply node and the second supply voltage exceeding thepredetermined threshold voltage value, generating the second outputvoltage value by driving the second output node of the voltage detectorto the third supply voltage.